1. Field of the Invention
This invention relates to the molding of soluble polymers to form molded products having precise tolerances, such as, contact lenses, surgical implants and other precisely shaped articles.
2. Background of the Related Art
Injection molding of molten plastics is a highly reproducible and cost effective method of manufacturing articles which must be precisely shaped to exacting tolerances. Some polymers, however, possessing highly desirable characteristics cannot be melted and injection molded. These include covalently cross-linked polymers, such as thermosets which cannot be melted or dissolved. They also include polymers with very strong physically cross-linked bonding which can be dissolved but cannot be melted. The physically cross-linked polymers include cellulose and its derivatives, aromatic polyamides, fully aromatic polyesters, polyacrylonitrile and ceratin block copolymers, including block copolymers having hydrogel character. The thermosets include hydrogels, which require special processing to be formed into precisely shaped products, such as contact lenses, surgical implants and other products.
The problems associated with the processing of thermosets are illustrated when dealing with covalently cross-linked hydrogels. Covalently crosslinked hydrogels, such as the polymer product of 2-hydroxyethylmethacrylate, crosslinked copolymers of acrylamide, vinylpyrrolidone, glycerylacrylate and methacrylic acid, are thermosets formed by copolymerization of their hydrophilic monomers with certain crosslinking monomers having two or more polymerizable double bonds. Since these polymers are covalently crosslinked, they are inherently insoluble and unmeltable. Therefore, they have to be polymerized in a final configuration, or polymerized into a blank which is subsequently shaped by mechanical means, such as lathing, grinding, polishing and the like.
Polymerization is a very sensitive process which is strongly influenced by several factors, including the presence of trace amounts of atmospheric oxygen, U.V. and other short-wave radiation, and by traces of impurities in the starting materials. Polymerization is also influenced by small, but unavoidable variations in the concentration of monomers and intiators of the polymerization process. These variables cause articles formed by polymerization molding techniques to vary from article to article and from batch to batch to a much greater extent then articles formed by injection molding techniques. Also, polymerization is accompanied by a decrease in volume, since almost all polymers have a higher density than the parent monomers. The decrease in volume during polymerization as a rule, is in the range of 10 to 20%. These factors make the polymerization and precise molding of products, directly in molds, a very costly procedure which is difficult to control and which results in a great deal of waste.
These difficulties become apparent in the manufacture of soft contact lenses which require very well defined surfaces. The shapes of the contact lens surface are extremely critical at the optical zones and edges, and must be free from surface defects, overflows, sharp edges or protrusions. Accordingly, in order to polymerize soft contact lenses into a final configuration, the mold has to have a fully enclosed, rigid molding cavity. Due to the factors outlined above however, and especially since the volume of the polymer is decreased from about 10 to 20% during polymerization, bubbles, vacuales and other defects are found in the molded product.
Many of the problems in molding hydrogels into contact lenses were solved by various processes, some of which are described by Larson, et al in U.S. Pat. No. 4,680,336. Generally, these processes included various combinations of the following methods:
1. A lens is lathed from hard, anhydrous hydrogel ("xerogel") to form a blank prepared in a separate polymerization step. The xerogel lens is then swelled to its final size. This method is widely used, but is rather expensive. Also, the individually manufactured lenses exhibit variations from lens to lens.
2. A mixture of monomer is polymerized in an open mold which is spun along its vertical axis. The anterior surface of the lens is formed by its contact with the mold, while the posterior surface is formed into an approximately paraboloid shape by the interplay of surface and centrifugal forces. This method solves some of the problems very elegantly, but its use is restricted to limited symmetrical shapes and to certain hydrogels.
3. Polymerization of the hydrogel monomer is carried out in a mold with a relatively rigid central part and softer, collapsible edges. The collapse of the edges by outside pressure diminishes the volume of the mold cavity as polymerization proceeds. One disadvantage of this method is that the edges are deformed in an unpredictable manner and must be reshaped by lathing and polishing in a separate step. Because of this reshaping step, the lens in the mold has to be formed in a machinable, i.e., xerogel state. In most cases, the mold must be disposable and the xerogel cannot be allowed to stick to the walls of the mold.
The methods which use machining of the lens in the xerogel state have several disadvantages. First, xerogel increases its volume by swelling, so that all dimensions, tolerances and defects increase in size between the xerogel and hydrogel states. The magnitude of enlargement increases with increasing water content in the final lens. Second, each lens is made from individually polymerized pieces of hydrogel, whether prepared as a blank or as a semifinished molded lens. This introduces variations in several parameters during lens production.
Problems associated with the production of other shaped hydrogel articles are similar, though usually not so critical as in the manufacture of contact lenses.
Physically cross-linked hydrogels can be used to manufacture the same types of products as covalently cross-linked hydrogels, such as contact lenses, surgical implants and the like. As a rule, physically crosslinked hydrogels have superior properties than covalently crosslinked hydrogels. By contrast to covalently crosslinked hydrogels, physically crosslinked hydrogels are water swellable polymers in which the covalent crosslinks are replaced by strong physical interactions between polymer chains. Physically crosslinked hydrogels appear frequently in nature; in addition several synthetic or semisynthetic polymers of this kind have been produced, including: hydrophilic segmented polyurethanes, certain derivatives of cellulose, block copolymers of vinylacetate-vinylalcohol, block copolymers of acrylonitrile-acrylamide, based on partially hydrolyzed polyacrylonitrile, and various hydrogel derivatives to name a few.
The physically-crosslinked hydrogels have been used to manufacture fibers, membranes, coatings, powderous sorbents, beads and similar articles which do not require precision molding methods. However, prior to this invention physically crosslinked hydrogels could not be utilized to produce precise, complicated or thick-walled molded articles.
The manufacture of shaped articles, such as contact lenses or individually adjusted implants, using physically crosslinked hydrogels, all pose a number of specific problems. Although these hydrogels can be "melted" using solvents, plasticizers or melting aids, when the melt is cooled and solidified in the molding cavity, the polymer shrinks in volume, creating defects and variations in the size of the molded product. Attempts to compensate for shrinkage in the mold have not been successful. Similarly, attempts to utilize spin casting for physically crosslinked hydrogels by cooling the melts or thermoreversible gels, have not been successful.
Physically crosslinked hydrogels can be also processed by coagulating their solution using a coagulating fluid, typically water. This method is normally used, however, for production of membranes, fibers, tubings, coatings, beads and sponges. However, it has never been used to manufacture shaped articles requiring a precise or complicated shapes, such as contact lenses.
In order to effectively mold dissolved, physically crosslinked hydrogels, it is necessary to extract the solvent and coagulate the polymer without changing the volume or shape of the polymer while it is being molded. The polymer solution must be coagulated in the mold or else the resulting article would be distorted.
In addition, other polymers with strong physical bonding, including aramides (Kevlar.TM.) or polyacrylonitrile also have excellent mechanical properties, such as low weight, high thermal resistance and tensile strength. However, because these polymers cannot be melted, their uses have been limited to articles with shapes only obtainable by traditional solution processing methods, for example, fibers and membranes.
Prior to this invention however, there has not been a method to precisely mold physically crosslinked hydrogels. Accordingly, although physically crosslinked hydrogels have many superior properties over covalently crosslinked hydrogels, including better mechanical properties at high water content and better general processability, physically crosslinked hydrogels could not be used to form precision molded articles due to the lack of a suitable molding method.